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Chapter 15 Probability Models
The Equally Likely Approach(also called the Classical Approach)
Assigning Probabilities
If an experiment has N outcomes, then each outcome has probability 1/N of occurring
If an event A1 has n1 outcomes, then
P(A1) = n1/N
DiceYou toss two dice. What is the probability of the outcomes summing to 5?
There are 36 possible outcomes in S, all equally likely (given fair dice).
Thus, the probability of any one of them is 1/36.
P(the roll of two dice sums to 5) =
P(1,4) + P(2,3) + P(3,2) + P(4,1) = 4 / 36 = 0.111
This is S:
{(1,1), (1,2), (1,3), ……etc.}
We Need Efficient Methods for Counting Outcomes
Counting in “Either-Or” Situations• NCAA Basketball Tournament, 68
teams: how many ways can the “bracket” be filled out?
1. How many games?2. 2 choices for each game3. Number of ways to fill out the bracket:
267 = 1.5 × 1020
• Earth pop. about 6 billion; everyone fills out 100 million different brackets
• Chances of getting all games correct is about 1 in 1,000
Counting Example
Pollsters minimize lead-in effect by rearranging the order of the questions on a survey
If Gallup has a 5-question survey, how many different versions of the survey are required if all possible arrangements of the questions are included?
Solution
There are 5 possible choices for the first question, 4 remaining questions for the second question, 3 choices for the third question, 2 choices for the fourth question, and 1 choice for the fifth question.
The number of possible arrangements is therefore
5 4 3 2 1 = 120
Efficient Methods for Counting Outcomes
Factorial Notation:n!=12 … n
Examples1!=1; 2!=12=2; 3!= 123=6; 4!
=24;5!=120;Special definition: 0!=1
Factorials with calculators and Excel
Calculator: non-graphing: x ! (second function)graphing: bottom p. 9 T I Calculator Commands(math button)
Excel:Insert function: Math and Trig category, FACT function
Factorial Examples20! = 2.43 x 1018
1,000,000 seconds?About 11.5 days1,000,000,000 seconds?About 31 years31 years = 109 seconds1018 = 109 x 109
20! is roughly the age (according to some) of the universe in seconds
Permutations
A B C D EHow many ways can we choose 2
letters from the above 5, without replacement, when the order in which we choose the letters is important?
5 4 = 20
Permutations (cont.)
20)!25(
!5:
45!3
!5
)!25(
!52045
25
PNotation
Permutations with calculator and Excel
Calculatornon-graphing: nPr
Graphingp. 9 of T I Calculator Commands(math button)
ExcelInsert function: Statistical, Permut
Combinations
A B C D EHow many ways can we choose 2
letters from the above 5, without replacement, when the order in which we choose the letters is not important?
5 4 = 20 when order importantDivide by 2: (5 4)/2 = 10 ways
Combinations (cont.)
!)!(
!
102
20
21
45
!2!3
!5
!2)!25(
!525
52
rrn
nC
C
rnnr
ST 305 Powerball Lottery
From the numbers 1 through 20,choose 6 different numbers.
Write them on a piece of paper.
Chances of Winning?
760,38!6)!620(
!20
ies?possibilit ofNumber
important.not order t,replacemen
without 20, from numbers 6 Choose
620206
C
Example: Illinois State Lottery
balls) pong pingmillion 16.5 house, ft (1200
months) 10in second 1about (
165,827,25!6!48
!54
importantnot order t;replacemen
withoutnumbers 54 from numbers 6 Choose
2
654 C
North Carolina Powerball Lottery
Prior to Jan. 1, 2009 After Jan. 1, 2009
:
55!3,478,761
5!50!
:
42!42
1!41!
3,478,761*42
146,107,962
5 from 1- 55
1 from 1- 42 (p'ball #)
:
59!5,006,386
5!54!
:
39!39
1!38!
5,006,386*39
195,249,054
5 from 1- 59
1 from 1- 39 (p'ball #)
Most recent change: powerball number is from 1 to 35http://www.nc-educationlottery.org/faq_powerball.aspx#43
The Forrest Gump Visualization of Your Lottery Chances
How large is 195,249,054?$1 bill and $100 bill both 6” in length
10,560 bills = 1 mileLet’s start with 195,249,053 $1 bills
and one $100 bill …… and take a long walk, putting
down bills end-to-end as we go
Raleigh to Ft. Lauderdale…
… still plenty of bills remaining, so continue from …
… Ft. Lauderdale to San Diego
… still plenty of bills remaining, so continue from…
… still plenty of bills remaining, so continue from …
… San Diego to Seattle
… still plenty of bills remaining, so continue from …
… Seattle to New York
… still plenty of bills remaining, so …
… New York back to Raleigh
Go around again! Lay a second path of bills
Still have ~ 5,000 bills left!!
Chances of Winning NC Powerball Lottery?
Remember: one of the bills you put down is a $100 bill; all others are $1 bills.
Put on a blindfold and begin walking along the trail of bills.
Your chance of winning the lottery is the same as your chance of selecting the single $100 bill if you stop at a random location along the trail and pick up a bill .
More Changes
After Jan. 1, 2009 After Jan. 1, 2012 http://www.nc-educati
onlottery.org/powerball_how-to-play.aspx
:
59!5,006,386
5!54!
:
39!39
1!38!
5,006,386*39
195,249,054
5 from 1- 59
1 from 1- 39 (p'ball #)
Virginia State Lottery
969000,52!1!24
!25760,118,2
760,118,2
760,118,2!5!45
!50: 5Pick
125
550
C
C